For example, in a projector for an optical device used for image display like a liquid crystal projector or a DLP (Trademark) projector, a high intensity discharge lamp (HID lamp) is used. In such a projector, light is separated into the three primary colors of red, green, and blue, i.e., by a dichroic prism etc., so that a space modulation element provided for each color generates an image of each of the three primary colors. The optical paths thereof are merged by a dichroic prism etc., so as to display a color image.
FIG. 22A shows a schematic view of the structure of a filter. As shown in the figure, the filter which comprises a color wheel having transmission color areas of R, G, and B which are three primary colors (i.e., an R color area (A3r), a G color area (A3g), and a B color area (A3b)) is rotated. This filter, i.e., a dynamic color filter, generates light flux of the three primary colors one by one by letting light from a light source, pass therethrough. In synchronization therewith, a space modulation element is controlled, so that an image of each of the three primary colors is generated sequentially in a time dividing manner, thereby displaying a color image. When brightness is important, as shown in FIG. 23A, W, i.e., white, may be added to the three primary colors of R, G and B, so that the dynamic color filter has four colors, (an R color area (A4r), a G color area (A4g), a B color area (A4b), and a W color area (A4w)), so that images of the four colors are generated one by one thereby displaying a color image. Or, furthermore, a color image may be displayed, by providing much more color areas thereon, thereby reinforcing color expression capability.
In a discharge lamp lighting apparatus for lighting a discharge lamp as described above, while voltage called a no-load open circuit voltage is impressed to a lamp at start-up, high voltage is impressed to the lamp, in order to generate dielectric breakdown in an electrical discharge space, so that the discharge state changes from glow discharge to arc discharge, and finally, the discharge lamp lighting apparatus is operated so that stable regular lighting may be realized. The lamp discharge voltage which was low, for example, about 10V, immediately after shifting to the arc discharge, goes up gradually in connection with a temperature rise, and is stabilized at fixed voltage in a lighting steady state. Usually, such a discharge lamp lighting apparatus has a converter which adjusts an output of an input power supply to a lamp discharge voltage, in order to realize a predetermined input power supplied to the lamp thereby outputting the required lamp current. Moreover, lamp voltage, i.e., output voltage of the converter, is detected, and based on this information, target lamp current is determined according to a value of the quotient which is obtained by dividing, for example, the target electric power by the detection voltage.
As types of discharge lamp driving methods, there are a direct current driving method in which a lamp is turned on by a converter, and an alternating current driving method in which periodic polarity reversals are performed by additionally providing an inverter further in the downstream side of the converter. In the case of the direct current driving method, since the light flux from the lamp is like direct current, that is, it does not change with passage of time, basically, there is a big advantage that it can be similarly applied to both types of the above-described projectors. On the other hand, in the case of the alternating current driving method, there is an advantage that growth or consumption of electrodes of the discharge lamp may be controlled, by using the flexibility of polarity-reversal frequency, which the direct current driving method does not have. On the other hand, there are disadvantages, resulting from existence of polarity reversals, that is, overshoot, instantaneous light-out at the time of polarity reversals etc., occur, thereby causing a bad influence on a display image etc.
Usually, in the alternating current driving method, transient phenomena, such as a slow change in lamp current, overshoot and oscillation, occurs every polarity reversal, due to induction components, such as a capacitance component which exists in the downstream side of an inverter, or a starting circuit. Since such a phenomenon appears in form of changes, such as, instantaneous light-out of the light flux from a lamp, overshoot, and oscillation, etc. almost as they are, when such a driving method is applied to a time dividing type projector among the above projectors, timing at which images are generated one by one in the time dividing manner, do not match timing at which the polarity is reversed in an alternating current driving method of a lamp, that is, the change appears in a display image at beat frequency, so that, there is a problem that it is very offensive to eyes of viewers, depending on the beat frequency. In order to solve the problem, the color wheel rotation is synchronized with the polarity-reversal timing of the inverter. However, the transient phenomena, such as a slow change in lamp current, or conversely overshoot or oscillation, which occurs every polarity reversal, cannot be avoided. Since it is difficult to effectively use lamp light emission at the period at which this phenomenon occurs, timing of a polarity-reversal operation of the discharge lamp lighting apparatus is adjusted, usually, in order that polarity reversal takes place at a period where the colors of the dynamic color filter are switched.
On the other hand, in order to obtain high color-reproduction performance of a display image, it is important to adjust spectrum distribution of a light source lamp and a form of conversion to the color sequential light flux, upon use of the dynamic color filter. In the case of the color wheel, it is possible to improve a color-reproduction performance or to carry out a desired color-reproduction performance by adjusting angle distribution of R, G, and B areas of the color wheel, that is, the rate of a period per rotation during which light transmits through each color area, according to the spectrum of the lamp. Depending on circumstances, in addition to the R, G, B, white (W) may be added.
For example, when using a lamp which runs short of an R component, as shown in FIG. 22B, or FIG. 23B, the occupancy angle ratio of each of an R color area (A3r′, A4r′), a G color area (A3g′, A4g′), and a B color area (A3b′, A4b′) and further a W color area (A4w′) around a rotational axis is set to uneven, so that the angle of the transmission area of the R component is enlarged. As shown in FIG. 24A or FIG. 25A, which schematically show a waveform of lamp current (IL′), respectively, it is effective to make transmission time rate of the R component longer than those of the other colors. In addition, reference characters, such as “R”, “G”, “B”, and “W” in the waveform of the lamp current (IL′) shown in FIGS. 24A, 24B, 25A and 25B, etc., respectively show periods of the main colors R, G, B, and W of the generated light flux of a dynamic color filter, that is, an R color period (Tr), a G color period (Tg), a B color period (Tb), and a W color period (Tw). As described below, the same reference characters are used in other figures.
However, in case where a desired color-reproduction performance is obtained in such a method in a DLP system type projector, since the brightness for every color of each pixel of a display image is controlled by a duty cycle ratio control operation on each pixel of a space modulation element, there is a problem that it is difficult to carry out fine control of pixel gradation, in the color component whose light transmission time rate is reduced. In order to solve such a problem, for example, in Japanese Laid Open Patent (Tokuhyo) No. H08-505031, it is proposed that, in an image projection apparatus, a light source drive control unit which changes an output power of the light source, synchronizing with the color of an optical beam outputted from a color change unit is provided. Similarly, in Japanese Laid Open Patent No. 2004-526992, a color-display apparatus in which electric power having two levels corresponding to colors is supplied, is proposed. In short, the common feature of such technology is modulation in which a pulse is superimposed on lamp current corresponding to the filter color which is appearing in a projector. As shown in FIG. 24B, or FIG. 25B, shortage of the R component is supplemented by making the lamp current (IL′) at the R component transmission period, larger than that of the other colors.
However, as described above, even if transmission time rate is made longer so as to supplement the shortage of the color component, or even if modulation is carried out so that a pulse may be superimposed on the lamp current, there is a problem that has not been solved. Although in a discharge lamp, at least at time of arc discharge, electrons which jump out of a cathode due to thermionic emission reach an anode, and then unnecessary kinetic energy is released, between the electrodes of the discharge lamp, much more heat is generated, i.e., a thermal load is large, in the anode than the cathode since the released kinetic energy is transformed into heat energy. In a discharge lamp which is operated in an alternating current driving method, the relation of the cathode and anode is switched every time polarity reversal takes place between the two electrodes.
In conditions in FIGS. 24A, 24B, 25A and 25B, in order to compensate shortage of an R component, in the polarity-reversal phase corresponding to the R component, a period from polarity reversal to the following polarity reversal is set to long or the current value thereof is set to large. However, in the conditions in FIGS. 25A and 25B, the polarity-reversal phase corresponding to the R component appears in and is always limited to a period where the lamp current (IL′) is positive. Therefore, under the conditions, in one of the two electrodes of the discharge lamp operated in an alternating current driving method, which becomes an anode, larger heat is always generated threat than the other electrode, at the polarity-reversal phase corresponding to the R component, so that the thermal load of these electrodes become imbalanced. For this reason, for example, one of electrodes is consumed intensively, whereby there is a problem that the lamp life span becomes short as the whole.
On the other hand under condition in FIGS. 24A and 24B, the polarity-reversal phase corresponding to the R component appears when the lamp current (IL′) is positive, and when it is negative, alternatively. Therefore, under this condition, there is no problem that the thermal load unbalance of the electrodes occurs. The reason why there is a difference as to existence or non-existence of the problem between the former condition and the latter condition is that while on the former condition, the number of times of the polarity reversals in an operation of one cycle of a dynamic color filter in the discharge lamp-lighting apparatus is odd, on the latter condition it is even.
In addition, although the example in which a period from polarity reversal to the following polarity reversal is extended in the polarity-reversal phase corresponding to the R component, and the example in which lamp current is increased, are shown above, a period from polarity reversal to the following polarity reversal may be changed similarly in the polarity-reversal phases corresponding to the other color or two or more color components. The situation is the same even when the lamp current is changed by the modulation, or even when the length of time from polarity reversal to the following polarity reversal is changed in addition to the modulation of the lamp current.
For this reason, for example, as proposed in Japanese Laid Open Patent No. 2006-227440, in a discharge lamp lighting apparatus, the number of times of the polarity reversals in an operation of one cycle of a color wheel whose number of the colors is even, is controlled to odd number of times. Moreover, it is proposed that a way of modulation of lamp current is adjusted so as to restrict the conditions so that integrated current values of both polar in an operation of one cycle of the color wheel are substantially the same. However, in the proposed method, although there is no problem as to the unbalance of the thermal load of the electrodes, a lamp has to be turned on under the restricted condition which is out of the desired operation condition, so that there is a problem that flexibility of the lamp lighting operation conditions is restricted.